We propose to investigate the physiological function of cyclic nucleotides within visual photoreceptor cells by intracellular injection of these agents and observation of the physiological state of the cells. We will investigate the physiological function of horizontal cells in the vertebrate retina by receptive field measurements, observation of these cells' electrical characteristics and by correlation of the observed properties with the morphologic entities identified by procion-yellow injection and by Golgi silver-chromate impregnation.